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Full text of "Handbook Of Chemical Engineering - I"

CRYSTALLIZATION                                     405
facture from sugar cane because of the much larger amount and difficulty of removal of the impurities. The treatment of the beet juice is much more difficult than that generally applied to cane juice, although in cane factories making white sugar the process is practically the same as in beet factories. The boiling and treatment of thick juice is practically the same for beet factories as for cane factories so far as the first crops of crystals are concerned. The interest of this section in beet-sugar manufacture lies in the methods employed in getting a maximum, yield of sugar from the molasses. Owing to the quantity and nature of the impurities in beet juices the final beet molasses has a "purity" of approximately 50 per cent as against 25 to 30 per cent for final cane molasses. Moreover, the beet molasses is of such a character that it has little value, except by special treatment, for the uses to which cane molasses so readily lends itself. This being the case, coupled with the fact that on account of its high sugar content there is a much greater quantity of final beet molasses produced,than is the case in cane factories, it has been found necessary, in order to produce beet sugar successfully, to devise means of recovering as far as possible all the sugar in the molasses.
Three processes have been worked out and extensively used. In order of their success as chemical processes they are: (1) the osmose process in which the crystalliz-able materials in the molasses are separated from the non-cryscallizable by osmosis in cells with permeable diaphragms; (2) the Steffens or lime process, in which the sugar is precipitated as calcium trisaccharate; (3) the barium process, in which the sugar is precipitated as barium trisaccharate.
Purification by Osmosis.—The osmose process was the first commercially applied and was at one time extensively used in Europe though only a little in this country. It has been almost entirely superseded by the Steffens process. The development of the osmose process was a great step in advance, but it has two inherent objections which caused its disuse soon after the Steffens method was brought out. The first difficulty is that for successful osmosis relatively dilute solutions must be used, which increased largely the amount of evaporation necessary to recover the sugar. The second objection is that not only the sugar, but also all crystallizable salts in the molasses pass the diaphragms, so that while this process made it possible to recover a large amount of the sugar in the molasses, the concentration of salts prevented the crystallization of a great deal of the sugar, this being a peculiar property of many of the mineral salts commonly found in molasses. A third, though less important objection, common to both the osmose and Steffens processes, is that the sugar raffinose (melitose, Ci8H32-Oie + 5H2O), which is found in considerable amount in beets under some conditions, is separated along with the sucrose and remains to upset calculations and give trouble later. Nevertheless the process offers a means of separating crystalloids from colloids especially where a battery of dialyzers is used.
Calcium and Barium Sucrates.—The formation of insoluble compounds of sucrose with salts of the metals of the alkaline earths, calcium, barium and strontium has long been known. The use of salts of barium and strontium commercially was proposed and tried out in Europe'many years ago but failed to receive approval, perhaps largely on account of the fear of traces of the known poisonous salts in the finished product. The cost of the necessary salts and the difficulty of reconversion into usable form also had great commercial influence. The value